* uplink: Add a new flag to set the filepath of the file which is used for
saving the encryption key and rename the one that hold the encryption key and
establish that it has priority over the key stored in the file to make the
configuration usable without having a huge refactoring in test-sim.
* cmd/uplink: Adapt the setup subcommand for storing the user input key to a file
and adapt the rest of the subcommands for reading the key from the key-file when
the key isn't explicitly set with a command line flag.
* cmd/gateway: Adapt it to read the encryption key from the key-file or use the
one passed by a command line flag.
* pkg/process: Export the default configuration filename so other packages which
use the same value can reference to it rather than having it hardcoded.
* Adapt several integrations (scripts, etc.) to consider the changes applied in uplink and cmd packages.
